The science of nuclear energy
The science of nuclear energy

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The science of nuclear energy

2.3.6 Solution: transmutation

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I think one of the most exciting prospects to come out of recent research is how to deal with nuclear waste. You see, long term waste remains radioactive for tens of thousands of years, so how to deal with it is obviously a very thorny issue. At the moment, the only accepted thing to do is to bury it deep underground in geologically sealed sites. But there's an obvious problem with this. It simply sits there as a legacy for future generations.
Here in Grenoble in the southeast of France, they're working on how to transform long term waste into something which can be disposed of more effectively. Dr Ulli Koester is in charge of researching this process here. It's called transmutation.
So we can turn one element to another, so we can destroy long-lived radioactive waste by turning it with this transmutation into short-lived isotopes, which go away quickly.
Ultimately, what happens in any nuclear reactor is that by splitting atomic nuclei, an element is transformed into other, different elements. And what they do here is rather similar-- just accelerated. They take heavy elements that are radioactive for tens of thousands of years and split them into lighter ones that are radioactive for just tens, or hundreds of years.
Transmutation's an alchemist's dream. It's where people try to convert lead into gold, which is actually possible with a strong accelerator, but the gold price has to go a long way before it becomes interesting economically.
To perform this work, they need a specialised nuclear reactor. They then take a small piece of radioactive material - in this case amaricium 241 - and load it remotely into the reactor's core. Once deep inside, it's bombarded with a high flux of neutrons, triggering fission of as many nuclei in the waste as possible. So burning it up more completely.
So here we have 50 times higher neutron flux compared to a power reactor, which means we can accelerate the process by a factor of 50. Instead of waiting for 50 years for something to happen, we can shorten it down to one year.
And this blue light in the shielding water is a sign that transmutation is happening. It's called Cherenkov radiation, and it's created by the products released as one element is changed into another. After 50 days or so in the reactor, the americium, which had a half-life of 430 years, has been transformed into completely different elements.
Each peak represents a fingerprint for an individual isotope. If you find this peak, we can look it up, and we will find it is a decay of krypton 87, which has a much shorter half-life of a couple of hours, so it will decay away very quickly.
It's a process that can be applied to other, more toxic waste products, which can be radioactive for thousands of years. It's not yet a working solution for our nuclear waste problems, but it shows what might be possible if scientists are able to pursue wider options.
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The video shows the process of transmutation. Heavy fission products with long half-lives are bombarded with neutrons and split into smaller fragments with shorter half-lives.

Opinion on transmutation is mixed. It does provide a solution to the problem of storing the long-lived isotopes in radioactive waste. It is also possible that the process of transmutation could itself be used to generate electricity and future power stations could incorporate transmutation into their running. This would reduce the volume of long-lived isotopes that are produced by fission of uranium.

However, the technology is not able yet to deal with large amounts of waste in an economically viable way and the research in this field would be expensive. Also transmutation would itself generate low-level radioactive waste!

In any event, while transmutation may significantly reduce the long-term risk of the radioactive waste, it wouldn’t replace the need for storage. The next section considers the siting of this storage.


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